The present application relates generally to nuclear reactors; and more particularly to, a system for dampening the level of vibration experienced by system piping within a nuclear reactor pressure vessel.
One type of nuclear reactor, a conventional boiling water reactor (BWR) is shown in FIG. 1. During operation of the reactor, coolant water circulating inside a reactor pressure vessel (RPV) 10 is heated by nuclear fission produced in the nuclear fuel core 35. Feedwater is admitted into the RPV 10 via a  inlet 15 and a sparger pipe 20, which is adjacent a core spray line 105. The flows downwardly through a downcomer annulus 25, which is an annular region between RPV 10 and a core shroud 30.
The core shroud 30 is a stainless steel cylinder that surrounds the nuclear fuel core 35, which includes a plurality of fuel bundle assemblies 40 (only a few are illustrated in FIG. 1). A top guide 45 and a core plate 50 supports each of the fuel bundle assemblies 40.
The coolant water flows downward through the downcomer annulus 25 and into the core lower plenum 55. Then the water in the core lower plenum 55 flows upward through the nuclear fuel core 35. In particular, water enters the fuel bundle assemblies 40, wherein a boiling boundary layer is established. A mixture of water and steam exits the nuclear fuel core 35 and enters the core upper plenum 60 under the shroud head 65. The steam-water mixture then flows through standpipes 70 on top of the shroud head 65 and enters the steam separators 75, which separate water from steam. The separated water is recirculated back to the downcomer annulus 25 and the steam flows out of the RPV 10 and to a steam turbine, or the like, (not illustrated).
The BWR also includes a coolant recirculation system, which provides the forced convection flow through the nuclear fuel core 35 necessary to attain the required power density. A portion of the water is drawn from the lower end of the downcomer annulus 25 via recirculation water outlet 80 and forced by a recirculation pump (not illustrated) into a plurality of jet pump assemblies 85 (one is illustrated in FIG. 1) via recirculation water inlets 90. The jet pump assemblies 85 are typically circumferentially distributed around the core shroud 30 and provide the required reactor core flow. A typical BWR has sixteen to twenty-four inlet mixers 95.
As illustrated in FIG. 1, a conventional jet pump assembly 85 comprises a pair of inlet mixers 95. Each inlet mixer 95 has an elbow welded thereto, which receives pressurized driving water from a recirculation pump (not illustrated) via an inlet riser 100. A type of inlet mixer 95 comprises a set of five nozzles circumferentially distributed at equal angles about the inlet mixer axis (not illustrated in the Figures). Here, each nozzle is tapered radially inwardly at the nozzle outlet. This convergent nozzle energizes the jet pump assembly 85. A secondary inlet opening (not illustrated) is radially outside of the nozzle exits. Therefore, as jets of water exit the nozzles, water from the downcomer annulus 25 is drawn into the inlet mixer 95 via the secondary inlet opening, where mixing with water from the recirculation pump then occurs.
During RPV 10 operation, the flow through the sparger pipe 20 contains pressure fluctuations from various sources in the reactor system. These pressure fluctuations can have frequencies close to one or more natural vibration modes of the sparger pipe 20. The vibration modes experienced by the  sparger pipe 20 depends, in part on, on preload and welds on the lugs 110 (not illustrated in FIG. 1) of the sparger pipe 20. The welds may also serve to keep the sparger pipe 20 at a specific distance away from the core spray line 105. In addition to pressure fluctuations, there may be other sources of vibration that can have frequencies close to one or more natural vibration modes of the sparger pipe 20. When an excitation frequency is near the natural frequencies of the sparger pipe 20, the resulting vibration may crack the welds on the lugs 110 and remove the preload from the sparger pipe 20. This can result in loss of the indication of core flow, which may require shutdown of the RPV 10.
There are a few possible problems with the currently known systems for dampening the vibration. Currently known systems involve re-welding the lugs 110, which may lead to a repeat failure. These systems generally require longer installation time and expose operators to longer period of radioactivity.
For the aforementioned reasons, there is a need for a new system for dampening the vibration experienced by the sparger pipe 20. The system should not require welding. The system should reduce the installation time and lower operator exposure to radioactivity.